Information may be recorded by exposure of a portion of an optical recording medium to a recording light beam thereby changing its local optical properties. The simplest such recording medium is a layer of a light absorptive material overlying a substrate. The information is recorded by locally melting or ablating the recording medium to form an opening therein. The presence of the opening results in a local change in the transmission and reflectivity of the recording medium which is detected during readout.
Spong in U.S. Pat. No. 4,097,895, issued June 27, 1978, disclosed an optical recording medium which comprises a light reflective layer which is coated with a light absorptive layer, wherein the thickness of the absorptive layer is chosen so that the reflectivity of the recording medium is reduced. A focussed, modulated light beam directed at the recording medium melts or ablates an opening in the absorptive layer, thereby changing the reflectivity of the recording medium. During readout the difference in reflectivity between unexposed and exposed portions of the recording medium is detected optically and converted into an electrical signal representative of the information so recorded.
Bell, in U.S. Pat. No. 4,216,501, issued Aug. 5, 1980, has disclosed a trilayer optical recording medium having a transparent spacer layer interposed between the reflective and the absorptive layers of the Spong optical recording medium. The thickness of the absorptive layer is so related to the thickness of the spacer layer and to the optical constants of the reflective, spacer and absorptive layers so as to reduce the optical reflectivity on the recording medium. Energy absorbed from a focussed, modulated light beam ablates, melts or otherwise changes the optical properties of the absorptive layer, thereby changing the reflectivity of the recording medium. The trilayer optical recording medium permits the use of a broader class of materials in a low reflectivity recording medium than that provided by the recording medium disclosed by Spong.
Bloom et al. in now U.S. Pat. No. 4,315,269, has disclosed a thick protective overcoat layer for an optical recording medium. The thick overcoat layer overlies the absorptive layer of the optical recording medium and keeps dust or other particles which settle on the structure far removed from the focal plane of the recording lens so that the influence of such particles on the recording and readout is considerably reduced.
However, the effect of the overcoat layer on the recording of information in a recording medium is to reduce the recording sensitivity, since the overcoat provides a thermal sink for energy absorbed in the absorptive layer. Thus, more energy must be supplied to the absorptive layer in order to raise its temperature to the temperature at which a change the absorptive layer, such as an opening, can occur. For certain absorptive materials the overcoat layer may also inhibit the formation of a deformation because of mechanical constraints on the movement of the absorptive material during the recording process.
Thus, it would be desirable to have an optical recording medium having a thick overcoat layer which maintains its beneficial aspects while eliminating the undesirable side effects of the overcoat layer of Bloom et al.